Elevated temperature method to adhere wear resistant polymer coating to ceramic article

ABSTRACT

A method is disclosed to provide a ceramic article with a protective coating which resists mechanical wear at elevated temperatures up to 500° C. and higher. The coating is applied by first chemically depositing the oxide product layer of a transition metal element on the uncoated ceramic surface and thereafter vapor depositing an adherent organic polymer lubricating film on the oxide product layer. Ceramic materials provided with such protective coating include silicon carbide, silicon nitride and vitreous silica.

This is a continuation-in-part of application Ser. No. 07/732,921, filedJul. 19, 1991, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to means enabling a ceramic article tobe provided with an adherent organic polymer lubricating film whichresists mechanical wear of the coated article at elevated temperaturesup to 500° C. and higher, and more particularly, to affording suchprotection to a variety of ceramic articles including both crystallineand vitreous ceramics.

Lubrication is a well recognized means to reduce friction and wearbetween surfaces in dynamic contact. Two major areas for which improvedlubricants are needed for continued progress are metal forming andtransportation. Better metal forming capabilities to minimize machiningand grinding require lubrication techniques and lubricants that can beused effectively at temperatures approaching the melting points of themetals now employed. In transportation, one of the most productive areasfor increasing energy efficiencies is often referred to as the"adiabatic" engine wherein temperatures range from 500° C. and 1100° C.making the selection of lubricants and means of lubrication stilldifficult. A known technique for lubricating at such temperatures is theuse of solid lubricants in the form of plasma sprayed coatings of themetals and ceramics being employed. More recent developments whereby anadherent solid polymeric lubricating film is deposited on a ferrousmetal surface to afford such protection are reported in technicalpublications entitled "In Situ Formation of Solid Lubricating Film fromConventional Mineral Oil and Ester Solid Duda, E. E. Graham and E. E.Klaus, ASLE Proceedings, 3rd International Conference on SolidLubrication, ASLE SP-14 1984 and "Lubrication from the Vapor Phase atHigh Temperatures", authored by E. E. Graham and E. E. Klaus, ASLEtransactions volume 29, no. 2, pages 229-234 (1986). As described insaid technical publications, the metal surfaces are deemed to have acatalytic effect upon the vapor phase reactants whereby surfacepolymerization of said reactants takes place to produce the protectivefilm. Possibly the absence of comparable metal catalytic agents inceramic materials has prevented the formation of the protective film insuch manner. More particularly, a vapor phase deposition of the samereactants under the same process conditions has thus far only producednon-adherent surface deposits affording no substantial protection to theunderlying ceramic substrate.

Silicon carbide and silicon nitride ceramics are now being studiedextensively for high temperature engineering applications as aboveindicated, to include advanced heat engines and gas turbines. Bothceramics are crystalline materials with silicon carbide being a veryhard material which is both corrosion and thermal resistant, is lighterthan steel and exhibits a high thermal conductivity and low thermalexpansion whereas silicon nitride is characterized by low thermalexpansion, excellent corrosion resistance and high temperaturestability. On the other hand, both of said ceramic materials undergosignificant mechanical abrasion when subjected to the dynamic wearconditions being experienced with such product applications. Vitreoussilica or fused quartz represents still a different type ceramicmaterial experiencing aggravated physical wear if subjected to dynamicconditions at elevated temperatures. Accordingly, it remains desirableto produce an effective means enabling various products formed witheither crystalline or vitreous ceramic materials to be provided with aprotective coating resistant to dynamic wear conditions such as abrasionand adhesive wear up to elevated temperatures of at least 500° C.

It is one object of the present invention, therefore, to provideeffective means whereby an adherent lubricating film can be deposited onvarious ceramic substrates affording resistance to mechanical wear ofthe coated article at elevated temperatures up to at least 500° C.

It is another object of the invention to provide novel surface treatmentmeans enabling a treated ceramic surface to be thereafter provided withan adherent wear resistant coating.

Still another object of the present invention is to provide a novelmethod for surface treatment of the ceramic article so as to enablesubsequent deposition of a wear resistant protective coating on thetreated ceramic surface.

A still further object of the present invention is to provide a novelmethod enabling a wear resistant protective coating to be formed on aceramic article

These and still further objects of the present invention will becomeapparent upon considering the following detailed description of thepresent invention.

SUMMARY OF THE INVENTION

Novel means have now been discovered to modify the surface of a ceramicarticle for deposition of an adherent solid organic polymer film thereonwhich resists mechanical wear of the ceramic article at elevatedtemperatures. More particularly, activating metal ions are firstdeposited in a particular manner upon the uncoated ceramic surfaces forimproved formation and adherence thereto of a subsequently vapordeposited solid organic polymer lubricating film which resistsmechanical wear of the ceramic article at elevated temperatures up to atleast 500° C. Metal compounds of a transition metal element selectedfrom the Periodic Table of Elements such as iron or tin serve as thesource of the activating metal ions in the present method. The uncoatedceramic surfaces are first chemically pretreated by contact with saidactivating metal compounds at elevated temperatures of at least 300°causing its conversion to an oxide product which is then deposited as asolid layer containing the activating metal ions on the treated ceramicsurfaces. For example, the uncoated ceramic article can be heated totemperatures of at least 300° C. for exposure thereat to a vaporizedsource of the activating metal compound and which can include furtherconditioning of the activated ceramic surface by subsequent exposure toambient atmospheric conditions. Alternately, the uncoated ceramicsurface can be activated by heating to the aforementioned elevatedtemperatures and subjected thereafter to a liquid dispersion of theactivating metal compound, such as by immersion or spraying to includeelectrostatic spraying. A continuous film of the activating metal ionsis chemically deposited in the foregoing manner on various ceramicsubstrates which can then optionally be further conditioned to promotecoating adherence by exposure to the atmosphere for time periods rangingfrom a few minutes to hours in duration.

A tenacious polymer lubricating film can now be directly applied to theoxide product layer in a particular manner. Specifically, the pretreatedceramic surfaces are thereafter exposed to a vaporized polymer formingorganic reactant selected from the class of petroleum hydrocarbonCompounds and synthetic lubricants at elevated temperatures of at least300° C. whereby an adherent solid organic polymer film is produced onthe treated surface which is visually observed to be substantiallydevoid of the underlying oxide product and found to resist mechanicalwear of the ceramic article at elevated temperatures of at least 500° C.While the influence of the activating metal ions on polymer surface filmformation has not been fully investigated at the present time, it isbelieved attributable at least in part to a formation of furtherreactive organic moieties in the gas phase of the particular reactionmedium herein being employed. The selected organic reactant materialscan be further characterized as exhibiting substantial vapor pressure atthe 300°-800° C. range herein employed for vapor deposition along withrelative chemical inertness and thermal stability at lower temperatures.Accordingly, petroleum hydrocarbon compounds such as dodecane andmineral oils can be used as well as aromatic type compounds. Suitablesynthetic Iubricants include polybutenes, diesters, polyglycols,chlorinated hydrocarbons, phosphate esters, silicate esters and thelike. A preferred class of esters include monobasic acid esters such asneopentylpoyolester, dibasic acid esters such as di-2-ethyl-hexylsebacate and phosphate esters to include tricresyl phosphate (TCP) andtripenyl phosphate.

In one representative embodiment, a satisfactory friction resistantpolymer coating has been applied to one or more surfaces of a siliconcarbide or silicon nitride block sample by a method wherein (a) theuncoated block sample is initially preheated in air to approximately300° C. or higher, (b) the preheated sample is next immersed in a liquiddispersion of the activating metal ion maintained at ambient temperatureto form a continuous surface film of the activating metal ion on thetreated sample, (c) the treated sample then allowed to dry at ambienttemperature for time periods ranging from a few minutes to twenty-fourhours whereupon conditioning of said surface film is found to promoteadherence of the subsequently applied polymer coating, and (d) theconditioned sample next placed in a suitable apparatus for deposition ofthe final coating on the now metal ion activated ceramic surface with avaporized polymer-forming organic reactant at temperatures in the rangefrom approximately 300° C. up to approximately 800° C. under slightlyoxidizing atmospheric conditions. Various metal ion liquid dispersionsemployed to form a continuous film on the ceramic sample prior to vapordeposition of the friction resistant polymer coating in theaforementioned manner have included liquid tin tetrachloride and ferricion solutions such as ten grams ferric acetylacetonate dissolved in 100milliliters methanol and twenty milliliters trichloroethylene as well asthree hundred grams ferric chloride hydrate dissolved in 100 millilitersmethyl alcohol and twenty milliliters hydrochloric acid. A still furtherpreferred chemical pretreatment medium comprises five grams ferricacetylacetonate dissolved in ninety milliliters of methylene chloride.Representative conditions forming the polymer coating within the coatingapparatus comprise exposing the preheated surface activated sample to acontinuous nitrogen gas stream containing approximately one-five volumepercent vaporized tricresyl phosphate and which may still furthercontain approximately one-five volume percent oxygen. A suitable coatingapparatus in which the illustrated method can be carried out in suchmanner is further described in connection with the following preferredembodiments.

In a different representative method of producing a satisfactoryfriction polymer according to the present invention, the ceramic surfaceis chemically treated prior to vapor deposition thereon of the polymercoating by means of electrostatically spraying a liquid dispersion ofthe activating metal ions (such as employed in the preceding embodiment)on the bare ceramic surface after be preheated to a temperature of atleast 300° C. While further processing of the ceramic article to producethe final polymer coating can thereafter proceed in the same manner asdescribed for said preceding embodiment, it is expected that variationstherefrom in the vapor deposition can produce comparable results. Forexample, a substitution of the other vaporizable polymer forming organicreactant previously mentioned should enable formation of the polymercoating under the same or similar reaction conditions. It can be furtherexpected that routine variation in the procedure herein illustrated forchemical pretreatment of the ceramic surface to include preheatingtemperatures, solution concentration and any further conditioning timeperiod can likewise influence the nature and extent of the polymercoating obtained with such substitution of organic polymer formingreactant in such reaction medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a representative coating apparatus for vapor depositionof a solid organic polymer lubricating film according to the presentinvention.

FIGS. 2a and 2b are microphotographs depicting polymer lubricating filmformation on hot pressed silicon carbide ceramic in the FIG. 1 coatingapparatus.

FIG. 3 is a microphotograph depicting polymer lubricating film formationon hot pressed silicon nitride ceramic in the FIG. 1 apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, there is depicted in FIG. 1 a representativecoating apparatus 10 which can be employed to form the organic polymerlubricating film according to the present invention on one or moresurfaces of a stationary ceramic article such as a body of siliconnitride, silicon carbide or vitreous silica. Coating apparatus 10includes a preheating chamber 12 and a vapor deposition chamber 14 whichare physically spaced apart to enable a vent passageway 16 therebetween.Preheating chamber 12 is provided with conventional heating means (notshown) as well as conduit means 18 enabling a nitrogen gas steam 19 tobe circulated therethrough at a typical rate of approximately twohundred cubic centimeters per minute. The nitrogen gas stream can beprovided within a stainless steel tube member 20 as shown which furtherprovides suitable containment means in which to house the ceramicarticles while being preheated. To further illustrate such means, threequarter inch diameter stainless steel tubing can be used in which toprocess ceramic articles having physical dimensions of0.25"×0.25"×0.125". As further shown in the drawing, a push rod member22 is provided through an entrance opening of said tubing member inorder to facilitate handling of the ceramic articles. Further chemicalpretreatment of said ceramic articles as hereinbefore defined in orderto provide one or more activated surfaces upon which the polymer film issubsequently formed is carried out with separate conventional apparatusmeans (not shown) immediately upon removing the heated ceramic articlesfrom the preheating chamber. Following the hereinbefore specifiedchemical surface activation treatment, the ceramic articles are nexttransferred to the vapor deposition chamber 14 which is maintained at atemperature in the approximate range 300°-800° C. with conventionalheating means (not shown). Vapor deposition chamber 14 includes anopen-ended ceramic combustion tube 24 connected at one end to anotherstainless steel tube member 26 enabling passage of a suitable polymerforming organic reaction medium into the combustion tube. Moreparticularly, stainless steel tube member 26 can have a T shapedconstruction packed with glass wool 28 for enhanced vapor mixing whilefurther being heated with a conventional heating tube 30. A liquidmedium 32 containing the polymer forming reactant such as liquidtricresyl phosphate is introduced into the tube member 26 at acontrolled rate so as to become vaporized in a nitrogen carrier gasstream 34 also therein introduced. To still further illustrate typicalconditions for vapor phase introduction of a suitable reaction mediuminto the vapor deposition chamber, the tricresyl phosphate reactionmixture herein being illustrated can employ from about one-five volumepercent vaporize tricresyl phosphate in the heated nitrogen gas streamand with the introduction temperature of said gaseous reaction mixturebeing maintained at approximately 300° C.

FIGS. 2a and 2b represent microphotographs obtained by conventionalscanning electron microscope means at two thousand times magnificationfor different surfaces of hot pressed silicon carbide articles aftercoating in the FIG. 1 apparatus. The reaction conditions employed in thevapor deposition chamber of said apparatus to produce the respectivepolymer coatings consisted of exposing the ceramic surfaces to anitrogen gas stream containing approximately 1.5 volume percenttricresyl phosphate for approximately three minutes at approximately700° C. The polymer coating shown in FIG. 2a was formed after activatingthe ceramic surface with a ferric acetylacetonate chemical pretreatmentas hereinabove defined and proved sufficiently adherent in protectingthe underlying ceramic surface from mechanical abrasion at temperaturesof at least 500° C. As distinct therefrom, the polymer coating depictedin FIG. 2b was formed without prior activation of the ceramic surfaceaccording to the present invention and was found to have such limitedadherence to the ceramic surface that it could easily be blown off.

In FIG. 3 there is depicted another photomicrograph again obtained byconventional scanning electronic microscope means but at a one thousandtimes magnification. This photograph depicts an adherent frictionpolymer coating deposited on a hot pressed silicon nitride ceramic afterchemical activation of the ceramic surface according to the presentinvention. In accordance therewith, the previously disclosed chemicalpretreatment with ferric acetylacetonate solution was employed toproduce a brown-orange film on the uncoated ceramic surface and thepolymer coating thereafter deposited on the treated surface atapproximately 750° C. with a nitrogen gas stream containingapproximately 1.5 volume percent tricresyl phosphate for an exposureperiod of approximately three minutes.

It will be apparent from the foregoing description that a broadly usefuland novel means has been provided to protect various type ceramicarticles from mechanical abrasion at elevated temperatures up to atleast 500° C. It is contemplated that modifications can be made in thespecific organic polymer coatings and methods for obtaining suchcoatings than herein illustrated, however, without departing from thespirit and scope of the present invention. For example, chemicalpretreatment of the ceramic surface with activating metal ions otherthan iron and tin are contemplated as well as employment of vaporizedpolymer forming organic reactants other than tricresyl phosphate, all tothe extent above further specified. Likewise, it is also contemplatedthat similar protective coatings can be applied to still other ceramicmaterials now being developed for high temperature dynamic productapplications, such as various stabilized zirconia ceramics.Consequently, it is intended to limit the present invention only by thescope of the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A method for applying a coating to a ceramicarticle providing improved resistance of the coated ceramic article tomechanical wear at elevated temperatures of 500° and above whichcomprises:(a) chemically treating the uncoated ceramic surface atelevated temperatures of at least 300° C. with a metal compound of atransition metal element which is converted to an oxide product of saidtransition metal element and deposited on the ceramic surface, and (b)exposing the treated ceramic surface to a vaporized polymer-formingorganic reactant selected from of petroleum hydrocarbon compounds andthe group of synthetic lubricants consisting of polybutenes, diesters,polyglycols, chlorinated hydrocarbons, phosphate esters and silicateesters at elevated temperatures of at least 300° C. whereby an adherentsolid organic polymer is produced on the treated surface which issubstantially devoid of said oxide product and resists mechanical wearof the ceramic article at elevated temperatures of at least 500° C. 2.The method of claim 1 wherein the ceramic article is a vitreous metaloxide.
 3. The method of claim 1 wherein the ceramic article is siliconcarbide.
 4. The method of claim 1 wherein the ceramic article is siliconnitride.
 5. The method of claim 1 wherein the treated ceramic surface isfurther conditioned by exposure to atmospheric conditions beforeexposure to the vaporized reactant.
 6. The method of claim 1 whereinexposure of the treated ceramic surface to the vaporized reactant atelevated temperatures is carried out under atmospheric conditionscontaining a minor concentration of oxygen.